1. Field of the Invention
The present invention relates to display panel devices and control methods thereof, and particularly to a display panel device using current-driven luminescence elements, and a control method thereof.
2. Description of the Related Art
Image display devices using organic electroluminescence (EL) elements have been known as image display devices using current-driven luminescence elements. Organic EL display devices using luminescence-producing organic EL elements do no require backlight necessary for liquid crystal display devices, and are suitable for flattening of display apparatuses. Furthermore, having no limitation on a view angle, the organic EL display devices are expected for practical use as next-generation display devices. The organic EL elements used in the organic EL display devices allow luminance of each of the organic EL elements to be controlled according to a current value of a current flowing thereinto, which differs from liquid crystal cells each of which is controlled according to a voltage to be applied thereto.
In a usual organic EL display device, organic EL elements serving as pixels are arranged in a matrix. An organic EL display is called a passive-matrix organic EL display, in which organic EL elements are provided at intersections of row electrodes (scanning lines) and column electrodes (data lines) and voltages corresponding to data signals are applied to between selected row electrodes and the column electrodes to drive the organic EL elements.
On the other hand, switching thin-film transistors (TFTs) are provided at intersections of scanning lines and data lines, connected to gates of driving elements, and turned on through selected scanning lines, and then data signals are inputted to the driving elements via signal lines. An organic EL element driven by a driving element is called an active-matrix organic EL display device.
Unlike the passive-matrix organic EL display device in which the organic EL elements connected to each of the row electrodes (scanning lines) produce luminescence only in a period during which each row electrode is being selected, because the active-matrix organic EL display device allows the organic EL elements to produce luminescence until next scanning (selection), the active-matrix organic EL display device does not cause a decrease in luminance of a display even when the number of scanning lines increases. Thus, the active-matrix organic EL display device can be driven at a low voltage, thereby achieving less power consumption.
Patent Reference 1 (Japanese Unexamined Patent Application Publication no. 2005-4173) discloses a circuit configuration of a pixel unit included in an active-matrix organic EL display device.
FIG. 17 is a circuit configuration diagram of a pixel unit included in the conventional organic EL display device described in Patent Reference 1. A pixel unit 500 in the figure is configured by a simple circuit element which includes: an organic EL element 505 having a cathode connected to a negative power line (voltage value is VEE); an n-type thin-film transistor (n-type TFT) 504 having a drain connected to a positive power line (voltage value is VDD) and a source connected to an anode of the organic EL element 505; a capacitor element 503 that is connected between a gate and the source of the n-type TFT 504 and holds a gate voltage of the n-type TFT 504; a third switching element 509 that causes both terminals of the organic EL element 505 to have a substantially same potential; a first switching element 501 that selectively applies a video signal via a signal line 506 to the gate of the n-type TFT 504; and a second switching element 502 that initializes a gate potential of the n-type TFT 504 to a predetermined potential. The following describes luminescence operation of the pixel unit 500.
First, the second switching element 502 is turned on with a scanning signal provided from a second scanning line 508, and the n-type TFT 504 is initialized by applying a predetermined voltage VREF supplied from a reference power line so that a current does not flow between the source and the gate of the n-type TFT 504 (S101).
Next, the second switching element 502 is turned off with another scanning signal provided from the second scanning line 508 (S102).
Next, the first switching element 501 is turned on with a scanning signal provided from a first scanning line 507, and a signal voltage supplied from the signal line 506 is applied to the gate of the n-type TFT 504 (S103). Here, a gate of the third switching element 509 is connected to the first scanning line 507, and becomes conductive concurrently with conduction of the first switching element 501. Accordingly, a charge corresponding to the signal voltage is accumulated in the capacitor element 503 without being influenced by a voltage across the terminals of the organic EL element 505. In addition, because a current does not flow into the organic EL element 505 while the third switching element 509 is being conductive, the organic EL element 505 does not produce luminescence.
Finally, the third switching element 509 is turned off with another scanning signal provided from the first scanning line 507, and a signal current corresponding to the charge accumulated in the capacitor element 503 is supplied from the n-type TFT 504 to the organic EL element 505 (S104). Here the organic EL element 505 produces luminescence.
With a series of above-mentioned operation, in one frame period, the organic EL element 505 produces luminescence at luminance corresponding to the signal voltage supplied from the signal line.